1. Field of the Invention
The present invention relates to lifting tongs for use in lifting heavy sections of cylindrical pipe.
2. Description of the Prior Art
In laying pipelines for use as water lines, sewer lines, oil lines and for other purposes the pipelines are assembled from sections of hollow, tubular, cylindrical lengths of pipe. Each section of pipe may weigh as much as several thousand pounds. Consequently, the sections can be lifted only with an extremely heavy duty crane. Also, the lifting cable of the crane must be provided with some means for gripping the cylindrical sections of pipe. While pipe sections may be lifted by means of encircling loops of cable which encompass the pipe sections, more typically the pipe sections are lifted and laid in place using some form of tongs. Some types of pipe tongs employed for this purpose are manufactured by Crescent Tongs, Inc., located at 1840 Coronado Ave., Long Beach, Calif. 90804 and sold under the trade designation Crescent Pipe Tongs.
Certain limitations exist in the types of pipe tongs which are currently available. Specifically, conventional pipe tongs are manufactured with jaws having gripping surfaces which are particularly suitable for use in cradling pipe sections having outer diameters falling within a rather limited range. Different pipe tongs are required to lift pipe sections of different diameters. For example, commercially available pipe tongs which are designed to lift pipe four inches in outside diameter cannot lift pipe having an outer diameter of fifteen inches. Moreover, conventional pipe tongs are dangerous to use for lifting pipe of a diameter which may vary only slightly from the optimum diameter for which the tongs are designed. When pipe tongs are utilized to lift pipe sections having a diameter greater than the diameter for which the tongs are designed, the tongs are likely to grip the circumference of the pipe over an insufficient arc. Consequently, the pipe can fall from the tongs, thus creating a serious risk of injury and damage. On the other hand, when pipe tongs are utilized to lift pipe of a diameter smaller than the diameter for which they are designed, the pipe is grasped too loosely, and can slide lengthwise relative to the tongs. When this occurs, the pipe will drop and likewise create a very serious risk of injury or damage.
A further limitation of conventional pipe tongs is that the gripping force with which the tong jaws grasp the wall of the pipe may be insufficient to adequately grip extremely heavy pipe, even though that pipe is of a diameter which the tongs are designed to lift. A heavy pipe section can thereby slip from the grasp of the jaws and drop.